Where the Teifi Marshes are, there used to be a tidal estuary. Then came the railroad, to transport the slate from the mines upstream, only the railroad slowly obstructed the tidal estuary with sediment and a wetland was born, which in turn blocked the railway and the mines were closing anyway.
So Teifi, as many other relics of the mining era was turned into a natural reserve, a lovely one indeed, where otters, water buffalos and even hazel dormice live. You can still see the slate among the vegetation and you can walk on the thin pointy crumbles.
Remember Lisa Simpson reading the Ecosystem of the Marsh. Marshes are amazing places, you just need the appropriate footwear. Teifi in particular is fascinating because of its wildlife and the ghosts of human intervention, embodied by the huge willow badger sculpture on the hill.